In general, it is desirable for a semiconductor device which is used as a switch to have low ON resistance and exhibit a small switching loss. Recently, an Insulated Gate Bipolar Transistor (IGBT) has become popular as a power semiconductor device having high breakdown strength (600 volts (V) or more, for example).
In the IGBT, when a positive bias with respect to an emitter electrode is applied to a gate electrode, an inversion layer is formed on a surface of a gate oxide film of a p-type base layer so that electrons are injected into an n−-type base layer. Due to such injection, the IGBT is brought into an ON state. When the IGBT is in a steady ON state, electrons and holes are stored in the n−-type base layer, and an ON voltage is reduced in accordance with an amount of the stored electrons and holes.
When the bias applied to the gate electrode is released with the IGBT in a turn-off switching state, the injection of electrons is stopped. Then, a depletion layer is formed at a pn junction between the p-type base layer and the n−-type base layer, and the depletion layer spreads toward a p+-type collector layer while discharging stored carriers. When a collector voltage becomes equal to a power source voltage, carriers remaining in the n−-type base layer in the vicinity of the p+-type collector layer generate a tail current thus increasing a turn-off switching loss. Accordingly, to reduce a switching loss, it is important to suppress the storage of carriers in a p+-type collector layer side of the n−-type base layer while the IGBT is in a steady ON state.